Bias for current transformer



May 14, 1963 1 K. DORTORT BIAS FOR CURRENT TRANSFORMER Filed Oct. 20, 1958 ICE-.5.

we mm mm. FO ED.

G W E T mm U MC NET EFFECTIVE NEGATIVE MAGNETIZING CURRENT CURRENT R D T RM MC RT R C OUEO RC SQUARE WAVE ' GENERATOR ADJUSTABL E PHASE SHI FTER MM m United States Patent 3,089,999 BIAS FOR CURRENT TRANSFORMER Isadore K. Dortort, Philadelphia, Pa., assignor to I-T-E Circuit Breaker Company, Philadelphia, Pa., a corporation of Pennsylvania Filed Oct. 20, 1958, Ser. No. 768,312. 6 Claims. (Cl. 32356) My invention relates to a highly accurate current transformer which utilizes a D.-C. bias and is a continuationin-part application of my copending application Serial No. 587,122, filed May 24, 1956, now Patent No. 2,883,603, entitled Cancellation of Break Step Current for Contact Converters and assigned to the assignee of the instant invention.

As is well known in the art, the accuracy of current transformers is limited by the magnetizing current of the transformer. That is, the ampere turns generated in the secondary winding is equal to the ampere turns in the primary winding minus the magnetizing current ampere turns. The later value is relatively independent of primary current magnitude so that it is not possible to make the secondary current vary in direct proportion to the primary current. Therefore, a certain inaccuracy is inherent in current transformers.

The principle of my invention is to substantially supply the current transforrner magnetizing current from an auxiliary source whereby the primary and secondary currents will vary linearly with respect to one another.

In a preferred embodiment of my invention, the current transformer core is formed of a material having a relatively rectangular hysteresis loop and the primary current is a unidirectional pulsating current whereby its magnetizing current can be supplied from a simple D.-C. source. However, if the primary current is an A.-C., then the bias could be an A.-C. pulse type bias which is reversed each half cycle so that the bias is in a proper direction for each half cycle. Furthermore, if the core is made of a relatively non-rectangular hysteresis loop material as by using a normal transformer iron core, then the bias could be formed of a current which initiates this relatively complex shape so that the magnetizing current of the current transformer is always supplied from the auxiliary bias circuit.

Accordingly, a primary object of my invention is to provide a highly accurate current transformer.

Another object of my invention is to provide a novel current transformer in which its magnetizing current is supplied from an auxiliary source.

A further object of my invention is to provide a novel current transformer in which an auxiliary source of power supplies the magnetizing current in the same direction as the primary current.

These and other objects of my invention will become apparent from the following description when taken in conjunction with the drawings, in which;

FIGURE '1 shows a rectifier circuit which requires a current transformer means of the type to which my invention is directed.

FIGURE 2 shows a modification of the circuit of FIG- URE 1.

FIGURE 3 schematically illustrates my novel current transformer construction.

FIGURE 4 illustrates the operating characteristic of the current transformer of FIGURE 3 for the case of circuit of the type of FIGURE 1 or 2.

FIGURE 5 specifically shows the current transformer of FIGURE 3 as applied to a circuit of the type of FIG- URE 1.

FIGURE 6 is a cross-sectional diagram of one possi- ICC ble manner in which the current transformer of FIGURE 3 can be constructed.

FIGURE 7 shows a second embodiment of my invention for an A.-C. type of current transformer.

Referring now to FIGURE 1, a commutating reactor core 20 is shown as having a winding 21 connected in series with a contact 22. Contact 22 may be mechanically driven into and out of engagement in synchronism with the A.-C. source 23 so as to deliver an average D.-C. voltage to the D.-C. load 24.

As may be seen with reference to Patent No. 2,693,569 issued November 2, 1954 entitled Commutating Reactor to Edward J. Diebold, as the load current 24 decreases through a zero value, the commutating reactor 20 will unsaturate to thereby provide a low current step within which the contact 22 may be opened.

So that this contact may interrupt a substantially zero current, I provide a current transformer having the secondary Winding 25 to measure the magnetizing current of the commutating reactor 20 and to thereafter impress this secondary output current across the contact 22 in series with a current limiting means 26. The current transformer having secondary 25 is more specifically a current transformer which could have a one-to-one turns ratio so that it will induce a current as shown by the arrow which is of the same shape and magnitude as the magnetizing current of commutating reactor winding 21 and flows through the contact 22 in a direction opposite to the magnetizing current. Hence, complete cancellation of the current through contact 22 may be obtained.

The current limiting valve 26 is required in the circuit so that after contact operation of contact 22 to a disengaged position, the reverse voltage appearing thereacross will be impressed across this current limiting valve 26. Hence, valve 26 is, as will be shown hereinafter, provided with the property of opposing current flow therethrough when the contact 22 is opened and still provides a substantially zero impedance circuit for current flow of smaller value in the same direction during the breakstep from the current transformer secondary winding 25.

FIGURE 2 shows a second embodiment of my novel invention wherein a current transformer seen generally at 27 is comprised of a core 28, a primary winding 29, and a secondary winding 34}. An auxiliary current transformer is then provided comprising the primary winding 31 and the secondary winding 32, this secondary winding 32 supplying the compensating current for the contact 22.

It may be desirable that a system such as that seen in FIGURE 2 be utilized wherein the turns ratio of the current transformer is not the exact one to one turns ratio described in conjunction with FIGURE 1, but a turns ratio which is so adjusted as to have an output current from secondary winding 32 which is of a slightly greater magnitude than the primary current through winding 29. This may be desirable in order to assure that a very slight positive current flows through the contact 22 during the break-stcp so that in the event of an arc during contact disengagement, this arc will be extinguished upon a subsequent passage through zero current. Moreover, the current transformer .27 is by this means approximately compensated for its magnetizing current.

A turns ratio which is slightly different from an exact one-to-one ratio would in a practical case be difiicult to obtain when utilizing a single current transformer as shown in FIGURE 1 in view of the relatively high currents conducted in mechanical rectifier or contact converter devices. That is to say, the number of turns of the primary and secondary windings will be substantially limited so that a first current transformer of FIGURE 2 which may be a one-to-one ratio current transformer may cause energization of winding 31 of an auxiliary current transformer, which winding may have a relatively high number of turns.

Hence, winding 32 may have a number of turns only slightly different from the number of turns of winding 3]; to thereby effect a turns ratio slightly less than one-toone between the primary winding 29 of the first current transformer and the second winding 32 of the auxiliary current transformer. Hence, the output current of secondary winding 32 may be slightly greater than the magnetizing current of the commutating reactor 2-3, which flows in current transformer primary windin g 29, and the net current through contact 22 will always be a very small positive value.

It is understood that in order to have an exact or substantially exact reproduction of the wave shape of the magnetizing current for the output of the compensating circuit, the current transformer utilized must be highly linear in the period during which that current flows.

In order to provide a current transformer having this high degree of linearity, I propose the use of a current transformer as may be seen in FIGURE 3 wherein the primary winding 33 which is the bus connection between the commutating reactor and the contact is surrounded by a core 34 constructed of highly saturable type material. The core 34 is preferably constructed to have a relatively small radial thickness to thereby effect a highly rectangular hysteresis loop for the core.

The current transformer shown in FIGURE 3 may be constructed in the manner shown in FIGURE 6 wherein the secondary bus is comprised of portions 33a and 33 which are connectible by the threaded current carrying connection 330. The core 34 which is preferably constructed of a wound tape is then positioned to encircle the current carrying protrusion of the bus portion 33a and carries therearound the single turn windings 35 and 36.

In FIGURE 3, the secondary winding 35 (which is equivalent to winding 25 of FIGURE 1) is to be connected to the contact 22 of FIGURE 1 through the current limiting means 26 and winding 36 is connected to be energized through the adjustable resistor 37 to a D.-C. voltage source as the battery 38. This type of construction offers a current transformer of an extremely high degree of linearity as may be understood with reference to FIGURE 4.

In FIGURE 4, the hysteresis loop of the core 34 of FIGURE 3 is seen with the magnetizing current of the commutating reactor superimposed thereupon, and seen as the curved line 39. The D.-C. bias winding 36 is then so energized as to magnetize core 34 in the same direction as it Would be magnetized by the flow of magnetizing current from the commutating reactor. Hence, the net effective magnetizing current of the current transformer assembly is the small difference between the bias current of Winding 36 and the magnetizing current of core 34, which is the very low value seen in FIGURE 4. Since the core 34 has a highly rectangular shape, then this effective magnetizing current, as is further seen in FIGURE 4, is relatively constant throughout the duration of unsaturation of the core 34. In view of the very low efiective magnetizing current of the current transformer of FIGURE 3, it is then realized that an extremely linear current transformer device is obtained; a characteristic which is very desirable for either of the em bodiments of FIGURES l or 2.

When the bias is equal to the uncorrected magnetizing current of the current transformer, the secondary current is exactly equal to the primary current. When the bias is smaller than the transformer magnetizing current, the net magnetizing current is a small positive value and the secondary current is smaller than the primary by that amount. If the bias is greater than the transformer magnetizing current, the net magnetizing current is negative,

and the secondary current is larger than the primary by that amount.

FIGURE 5 shows the type of current transformer of FIGURE 3 as being applied to the contact converter of FIGURES 1 and 2 and further shows a particular type of current limiting valve which comprises the resistor 40, reactor 41 and diode 42. This system operates in a manner similar to the system described in copending application Serial No. 490,319. Reactor 41 is charged by the inverse cycle current i which flows due to the inverse voltage across the contact 22 while it is open and will maintain a current flow in the forward direction of diode 42, this current being shown as the current i when the inverse voltage disappears, as when the contact closes. In the event of unsaturation of the commutating reactor core 20, the current transformer secondary winding 34 will cause a compensating current I}, to flow through the contact 22 in a direction to buck down the current i flowing through diode 42.

By providing the type of valve means shown in FIG- URE 5, it is now seen that during the flow of compensating current that this current is merely effective to buck down an existing current through a semi-conductor and in view of this, it will be a very small impedance. Hence, when the net contact current i which is the difference between the magnetizing current of commutating reactor winding 20 which flows through the contact 22 and the compensating current of winding 34, is broken, the circuit will have an extremely small impedance in parallel with the disengaging contacts and the voltage drop due to the compensating circuit in parallel with contacts 22 will be negligible.

As has been desecribed above, my novel invention may be applied to an A.-C. type of current transformer. This construction is schematically set forth in FIGURE 7 in which an A.-C. system including voltage source 50 and load 51 has a current transformer 52 connected therein for measuring the current of the system. Current transformer 52 is constructed of a core 53 which is preferably of a highly saturable type of material with primary winding 54, secondary winding 55 and a compensating winding 56 wound thereon. The secondary winding 55 is connected to an output load means 57 which could include an instrument for indicating current in the usual manner. The compensating circuit for energizing the compensating winding 56 includes an auxiliary voltage source 57 which could be derived from the main voltage source 56. The voltage of source 57 is connected through an adjustable phase shifter 58 and a square wave generator 59 so that a relatively square wave current is supplied to compensating winding 56. Since both the phase shifter 58 and square wave generator 59 are well known devices and their structure would be obvious to those skilled in the art, details of their construction are not given herein.

The core 53 of current transformer 52 preferably has a rectangular hysteresis loop similar to that illustrated in FIGURE 4. The true magnetizing current of this core is essentially a square wave current which is in phase with the developed voltage of the transformer. The phase relationship between the magnetizing current of the core and the primary current of winding 54 is determined by the power factor of the load 57 connected to core winding 55'. Generally, this power factor is fixed except under extreme load conditions and can be readily determined. However, the phase shifter 58 is preferably interposed between the auxiliary voltage source 57 and compensating winding 56 so that the phase of the compensating current is adjustably controlled to be in phase with the magnetizing current of the core 53.

The operation of the circuit of FIGURE 7 will be identical within each half cycle to the operation of the circuit of FIGURE 3. That is to say, when the ma netizing current of core 53 is positive, winding 56 will conduct a compensating current which Will serve as a source of magnetizing current for the core. Thus, the current in winding 54 and the current in winding 55 will be substantially directly proportional to one another. When the magnetizing current of core 53 reverses, the square Wave compensating current applied to winding 56 will reverse and the same compensating process described above will continue.

Clearly, the basic concept shown in FIGURE 7 may be modified for cases in which winding 56 will carry any shape current to compensate for a similar shaped magnetizing current of the current transformer core.

In the event the circuit is such that phase angle of load 57 is variable, it will be obvious to those skilled in the art that feed-back control means from load 57 to phase shifter 58 could be applied so that the proper phasing of the energization of winding 56 will be maintained with respect to the magnetizing current of the core.

In the foregoing, I have described my invention only in connection with preferred embodiments thereof. Many variations and modifications of the principles of my invention within the scope of the description herein are obvious. Accordingly, I prefer to be bound not by the specific disclosure herein but only by the appending claims.

I claim:

1. Means for reducing the effective magnetizing current of a current transformer; said current transformer comprising a core of saturable type material, a primary winding, a secondary winding and a D.-C. bias winding; said primary winding being energizable from a source of unidirectional pulsating current; said D.-C. winding being energizable from a D.-C. power source; said core of saturable type material having a relatively small radial thickness to effect a highly rectangular hysteresis loop for said core; said D.-C. bias winding and said primary winding being connected to magnetize said saturable core in the same direction during that portion of the primary wave where high accuracy is desired responsive to energization by their respective sources.

2. A compensating means for a current transformer; said current transformer comprising a magnetic core having a primary winding and a secondary winding thereon and a rectangular hysteresis loop; said secondary Winding having a current flow induced therein which is functionally related to the current flow through said primary winding; said compensating means including a winding wound on said magnetic core and a source of current for said winding to magnetize said core in the same direction as the current through said primary winding which is to be reproduced in said secondary winding; said compensating means generating the magnetizing ampere turns normally supplied by said primary winding said source of current supplying a constant current over any half cycle.

3. A compensating means for a current transformer; said current transformer comprising a magnetic core having a. primary winding and a secondary winding thereon; said secondary Winding having a current flow induced therein which is functionally related to the current flow through said primary winding; said compensating means including a current source, and winding means associated with said magnetizable core to generate ampere turns in said magnetizable core; said current source generating a current having substantially the same shape as the magnetizing current required for said magnetizable core; said winding means being constructed to have sufiicient turns to supply the same ampere turns that would be required of said primary winding in the absence of said compensating means.

4-. A coni ensating means for a current transformer; said current transformer comprising a magnetic core having a primary winding and a secondary winding thereon; said secondary winding having a current flow induced therein which is functionally related to the current flow through said primary Winding; said compensating means including a current source, and winding means associated with said magnetizable core to generate ampere turns in said magnetizable core; said current source generating a current having substantially the same shape as the magnetizing current required for said magnetizable core; said winding means being constructed to have sufficient turns to supply the same ampere turns that would be required of said primary Winding in the absence of said compensating means; said core being of relatively high permeability material having a rectangular hysteresis loop; said current source generating a relatively constant current.

5. A compensating means for a current transformer; said current transformer comprising a magnetic core having a primary winding and a secondary winding thereon and a rectangular hysteresis loop; said secondary winding having a current flow induced therein which is functionally related to the current flow through said primary winding; said compensating means including a winding wound on said magnetic core and a current source therefor to magnetize said core in the same direction as the current through said primary winding Which is to be reproduced in said secondary winding; said compensating means generating the magnetizing ampere turns normally supplied by said primary winding; said current flow through said primary winding being an alternating current; said magnetizing ampere turns generated by said compensating means being a square wave current.

6. A compensating means for a current transformer; said current transformer comprising a magnetic core having a primary winding and a secondary winding thereon and a rectangular hysteresis loop; said secondary winding having a current flow induced therein which is functionally related to the current flow through said primary winding; said compensating means being magnetically connected to said magnetic core to magnetizc said core in substantially the same direction as the current through said primary winding which is to be reproduced in said secondary Winding; said compensating means generating the magnetizing ampere turns normally supplied by said primary winding; said current flow through said primary winding being an alternating current; said magnetizing ampere turns generated by said compensating means being a square wave current; and phase shift means connected to said compensating winding for controlling the phase shift of said magnetizing ampere turns generated by said compensating means with respect to the induced voltage of said magnetizable core.

References Cited in the file of this patent UNITED STATES PATENTS 2,746,003 Wegener May 15, 1946 2,773,133 Dunnet Dec. 4, 1956 2,866,158 Petzinger Dec. 23, 1958 OTHER REFERENCES IBM Technical Disclosure Bulletin, August, 1958, p 

6. A COMPENSATING MEANS FOR A CURRENT TRANSFORMER; SAID CURRENT TRANSFORMER COMPRISING A MAGNETIC CORE HAVING A PRIMARY WINDING AND A SECONDARY WINDING THEREON AND A RECTANGULAR HYSTERESIS LOOP; SAID SECONDARY WINDING HAVING A CURRENT FLOW INDUCED THEREIN WHICH IS FUNCTIONALLY RELATED TO THE CURRENT FLOW THROUGH SAID PRIMARY WINDING; SAID COMPENSATING MEANS BEING MAGNETICALLY CONNECTED TO SAID MAGNETIC CORE TO MAGNETIZE SAID CORE IN SUBSTANTIALLY THE SAME DIRECTION AS THE CURRENT THROUGH SAID PRIMARY WINDING WHICH IS TO BE REPRODUCED IN SAID SECONDARY WINDING; SAID COMPENSATING MEANS GENERATING THE MAGNETIZING AMPERE TURNS NORMALLY SUPPLIED BY SAID PRIMARY WINDINGS; SAID CURRENT FLOW THROUGH SAID PRIMARY WINDING BEING AN ALTERNATING CURRENT; SAID MAGNETIZING AMPERE TURNS GENERATED BY SAID COMPENSATING MEANS BEING A SQUARE WAVE CURRENT; AND PHASE SHIFT MEANS CONNECTED TO SAID COMPENSATING WINDING FOR CONTROLLING THE PHASE SHIFT OF SAID MAGNETIZING AMPERE TURNS GENERATED BY SAID COMPENSATING MEANS WITH RESPECT TO THE INDUCED VOLTAGE OF SAID MAGNETIZABLE CORE. 